This invention relates in general to the investigation of the near-surface electronic properties of semiconductors and especially to the investigation of the electron affinity and the position of the conduction-band edge with respect to the Fermi level in single-crystal semiconductors. More particularly, this invention relates to an electron-beam scanning method and apparatus for investigating the uniformity of the foregoing near-surface properties with respect to surface locations and providing a visual presentation thereof.
The near-surface properties of semiconductors are of great importance in determining the suitability of the semiconductors for a particular application and for predicting the actual operational performance of semiconductor devices. Many of these near-surface properties manifest themselves for study and analysis in the electronic properties of the surface work function .phi. and its two components (a) the position of the conduction-band edge E.sub.c with respect to the Fermi level and (b) the electron affinity .chi.. The position of the conduction-band edge is governed by such band structure properties of the solid as the donor position, donor density, and ionization probability, while the electron affinity is strongly dependent on characteristics such as the surface stoichiometry, surface dipole layers, and Schottky barriers. It would be desirable to separate the work function into its two components because this will not only determine the various properties for a particular set of surface conditions, but will also quantitatively identify changes in work function as being caused by specific contributions from either or both of its components.
In the past, there was no singularly direct and reliable technique for experimentally determining these two components of the work function despite a great deal of interest in the area. Near-surface conductivity (and factors such as doping density associated with it) could only be inferred from surface potential measurements. Similarly, the electron affinity has been inferred from combining the results of separate procedures. For example, the electron affinity has been estimated from the temperature at which pore conductivity and crystal conductivity are roughly equal in oxide cathodes. It has also been derived by combining optical adsorption and photoconductivity measurements. These techniques, in addition to requiring separate experiments, also involve assumptions that give uncertainty to the estimated values, and are not generally applicable to all semiconductors.
Applicants' copending application (filed on the same day as this application) entitled "Method for Separating the Conduction-Band Edge and Electron Affinity in Semiconductors" discloses that low-energy electrons incident on a single-crystal semiconductor surface result in electron reflections corresponding to unallowed energy states in the conduction band where the Bragg condition n.lambda. = 2d exists (n being an integer, .lambda. being the DeBroglie wavelength, and d being the crystal lattice spacing normal to the surface). The relationship between energy of the incident electrons and the current collected by the semiconductor exhibits maxima and minima which are related to the position of the conduction-band edge E.sub.c. The work function, .phi., the conduction-band edge E.sub.c and the electron affinity .chi. can be determined from this relationship.
However, it is also desirable to investigate the uniformity of the surface work function and each of its components. In general, the work function of a semiconductor surface is not uniform throughout the surface, but may be composed of many small regions (commonly called patches) having different values of work function. A determination of the relative contributions of E.sub.c and .chi. to the work function, and the variations of these quantities over the surface will provide a large amount of information about the surface properties of the semiconductor under study.